1. Field of the Invention
The present invention relates to a circuit for driving a light emitting element and, more particularly, to a drive circuit for driving a light emitting element which emits light according to an element current. The present invention also relates to a current-control-type light emitting display using a light emitting element which emits light according to an element current.
2. Description of the Related Art
Current-control-type light emitting displays ordinarily have light emitting elements arrayed in matrix form and perform display control utilizing the phenomenon of emission of light from the light-emitting elements. A current-control-type light emitting display of a low current consumption can be realized by using light emitting elements having a high light emission efficiency. The current flowing per unit time in an active-matrix current-control-type light emitting display in particular is lower than that in a simple-matrix current-control-type light-emitting display or the like, and the active-matrix current-control-type light emitting display can display images at a low voltage and a low power consumption. In recent years, organic EL elements capable of emitting light at a low voltage and a low current have been put to use as light emitting elements in active-matrix current-control-type light emitting displays.
Organic electroluminescent (EL) elements can be driven at a low voltage and a low current. However, there are cases where a light-emitting element formed of an organic EL element does not emit light after forming films for the element due to a short-circuit defect which occurs between the anode and the cathode. This short-circuit defect can be removed by applying a voltage higher than a certain value and opposite in polarity to the voltage generated between the anode and the cathode at the time of light emission. In other words, a reverse bias voltage equal to or higher than a certain value is applied between the anode and the cathode of the light emitting element to cause a sufficiently large reverse current to flow through the light emitting element to insulate the short-circuit defect portion. The short-circuit defect can be removed in this way. In a current-control-type light emitting display using organic EL elements, a reverse bias voltage is applied to light emitting elements after film forming for the elements to cause a short-circuit defect, which has occurred, to disappear.
A short-circuit defect as well as that found in the above-described situation may occur in organic EL elements when the organic EL elements are operated by causing only a forward current to flow. Also in this case, the short-circuit defect can be removed by applying a reverse bias voltage to the element to restore the element to the normal light emitting condition. Also, there are cases where the life of organic EL elements is extended due to application of a reverse bias voltage to the elements in comparison with the case where only a forward current is caused to flow. Also from this viewpoint, it is desirable to apply a reverse bias voltage to organic EL elements.
There is described in Japanese Patent Application laid open No. 2003-122304 (reference 1) a known technique for applying a reverse bias voltage to light emitting elements in conventional current-control-type light emitting displays. According to the technique, a reverse bias voltage is applied to light emitting elements for the purpose of extending the life of the light emitting elements. Application of a reverse bias voltage to the light emitting elements in this technique is also effective in reducing short-circuit defects in the elements.
FIG. 1 shows a portion of a conventional display described in reference 1. Referring to FIG. 1, a source signal line 41 through which a signal current is supplied is connected to the drain of a first switching transistor 47c, and a first gate signal line 42 is connected to the gate of the first switching transistor 47c and to the gate of a second switching transistor 47b. The source of the first switching transistor 47c is connected to the drain of the second switching transistor 47b, the drain of a drive transistor 47a and the source of a third switching transistor 47d. The source of the drive transistor 47a is connected to an EL power supply line 45. The source of the second switching transistor 47b is connected to the gate of the drive transistor 47a and is also connected to the EL power supply line 45 via a storage capacitor 44.
A second gate signal line 43 is connected to the gate of the third switching transistor 47d and the gate of a fourth switching transistor 47e. The drain of the fourth switching transistor 47e is connected to a reverse bias power supply line 48. One of two electrodes of an EL element 46 is connected to the drain of the third switching transistor 47d and the source of the fourth switching transistor 47e, while the other electrode of the EL element 46 is connected to a power supply line 49.
In the technique described in reference 1, a voltage of L level is supplied to the first gate signal line 42 during a selection period in one frame period. Each of the second switching transistor 47b and the first switching transistor 47c is thereby made conductive. At this time, a voltage of H level is supplied to the second gate signal line 43 to set the third switching transistor 47d in a shutoff state. As a result, a current controlled according to the signal current supplied from the source signal line 41 is thereby caused to flow through the drive transistor 47a, and a voltage according to the signal current supplied from the source signal line 41 is generated at the gate of the drive transistor 47a and one end of the storage capacitor 44.
After the end of the selection period, the H-level voltage is supplied to the first gate signal line 42 to set each of the second switching transistor 47b and the first switching transistor 47c in a shutoff state. Since the second switching transistor 47b is set in the shutoff state, the voltage generated at the drive transistor 47a and one end of the storage capacitor 44 is held by the storage capacitor 44. At this time, if the voltage supplied to the second gate signal line 43 is at L level, the third switching transistor 47d is in the conductive state and the fourth switching transistor 47e is in the shutoff state. Consequently, the source-drain current in the drive transistor 47a supplied from the EL power supply line 45 flows into the EL element 46 via the third switching transistor 47d. 
If the voltage supplied to the second gate signal line 43 after the selection period is not at L level but at H level, the third switching transistor 47d is in the shutoff state, the fourth switching transistor 47e is in the conductive state, and no current flows from the EL power supply line 45 to the EL element 46. In this case, since the fourth switching transistor 47e is in the conductive state, a voltage supplied to the reverse bias power supply line 48 to apply a reverse bias voltage to the EL element 46 is applied to one of the two electrodes of the EL element 46. A voltage supplied to the power supply line 49 connected to the other electrode of the EL element 46 is ordinarily 0 V or a negative voltage. Therefore, a negative voltage lower than the voltage applied to the power supply line 49 is supplied to the reverse bias power supply line 48 to reduce the potential on the third switching transistor 47d/fourth switching transistor 47e side of the EL element 46 relative to the potential on the power supply line 49 side.
The technique described in reference 1 requires two negative power supplies if the voltage supplied to the power supply line 49 is negative, or one negative power supply if the voltage supplied to the power supply line 49 is 0 V. That is, the technique described above requires at least one negative power supply for application of a reverse bias to the EL element 46. Therefore, it is difficult to reduce the size and manufacturing cost of a display by using the technique. There is another problem as below. Since a negative voltage is applied to the reverse bias power supply line 48 and a voltage of H level is applied to the second gate signal line 43, an excessively high voltage corresponding to the sum of the H-level voltage and the absolute value of the negative voltage is applied between the gate and the source of the third switching transistor 47d, between the gate and the drain of the third switching transistor 47d, between the gate and the source of the fourth switching transistor 47e, and between the gate and the drain of the fourth switching transistor 47e. Therefore, gate insulation breakdown or degradation in electrical characteristics can occur easily in the third switching transistor 47d and the fourth switching transistor 47e. 
There is described in Japanese Patent Application laid open No. 2002-169509 (reference 2) another known technique for applying a reverse bias voltage to light emitting elements. According to the technique, a reverse bias voltage is applied to light emitting elements for certain purposes including the purpose of inhibiting life-shortening due to degradation in film quality of the elements. FIG. 2 shows a pixel circuit for a conventional display described in reference 2. In a pixel circuit 54 of a display 50 in FIG. 2, an external power supply 53 is connected to one end of an EL element 56 and the voltage of the external power supply is controlled to apply a reverse bias voltage to the EL element 56. More specifically, the voltage of the external power supply 53 and the voltage on the power supply line 55 are set in a relationship (external power supply 53)>(power supply line 55) to apply a reverse bias voltage to the EL element 56 and supply a reverse bias current to the EL element 56 via a second thin-film transistor 58 for controlling the value of a current supplied to the EL element 56 at the time of light emission.
When the reverse bias voltage is applied to the EL element 56 having a short-circuit defect, a large current several ten times larger than that at the time of light emission is caused to flow through the EL element 56. In ordinarily cases of causing a large current to flow through a device on/off controlled, e.g., a switching device, there is a need to set the size of the device large, for example, a need to set the channel width large in the case of a thin film transistor. In the construction shown in FIG. 2, therefore, the second thin-film transistor 58 as a current control transistor for controlling the current caused to flow through the EL element 56 at the time of light emission needs to have an increased size according to a current which is caused to flow in the reverse direction through the EL element 56 as a reverse current large enough to remove a short-circuit defect.
In the second thin-film transistor 58, however, the channel width cannot be sufficiently increased because the channel width is set to a value for accurately controlling the current supplied to the EL element 56 at the time of light emission. For this reason, the value of the reverse current supplied to the EL element 56 is limited by the second thin-film transistor 58. In the construction shown in FIG. 2, therefore, a sufficiently large reverse current cannot be caused to flow through the EL element 56 and it is difficult to remove a short-circuit defect. A sufficiently large potential difference may be set between the gate and the source of the second thin-film transistor 58 to compensate for the low current performance of the second thin-film transistor 58 and to cause a sufficiently large current to flow through a short-circuit defect portion in the EL element 56 at the time of application of the reverse bias. In such case, however, the voltage applied between the gate and the source is excessively high, there is a possibility of the gate-source voltage exceeding the withstand voltage to break the thin-film transistor, and the reliability of the light emitting display is reduced.
As described above, the technique described in reference 1 requires at least one negative power supply for application of a reverse bias voltage to EL elements and entails difficulty in reducing the size, manufacturing cost and power consumption of a display. Also, a negative voltage is applied to the reverse bias power supply line 48 and a voltage of H level is applied to the second gate signal line 43. Therefore, an excessively high voltage is applied to the third switching transistor 47d and the fourth switching transistor 47e and gate insulation breakdown and degradation in electrical characteristics occur easily.
According to the technique described in reference 2, a reverse bias voltage is applied through a current control transistor for controlling the current supplied to an EL element at the time of light emission, and no negative power supply is required. However, a large current necessary for insulating a short-circuit portion cannot be caused to flow through the current control transistor, and therefore, it is difficult to remove a short-circuit defect by this technique. The technique described in reference 2 also has a problem in terms of reduction in reliability in that when a large voltage is applied between the gate and the source of the current control transistor to cause a large reverse bias current to flow, destruction of the current control transistor or degradation in electrical characteristic cannot be avoided.